Cambodia’s Khmer Rouge trial entered a crucial phase on Wednesday with closing statements in the case of former regime leaders accused of masterminding one of the worst horrors of the 20th century.

More than three decades after the country’s “Killing Fields” era, the UN-backed court is moving closer to a verdict in the case of “Brother Number Two” Nuon Chea, 87, and ex-head of state Khieu Samphan, 82.

Lawyer Pich Ang, representing victims, described the Khmer Rouge as “one of the most heinous regimes history has ever known”.

Led by “Brother Number One” Pol Pot, who died in 1998, the communist Khmer Rouge wiped out up to two million people through starvation, overwork and execution in the late 1970s.

The trial, which began hearing evidence in late 2011, is widely seen as a landmark in the nation’s quest for justice.

The defendants deny charges of war crimes, crimes against humanity and genocide.

Around 500 Cambodians, including former regime cadres, watched the trial from the public gallery on Wednesday.

Nuon Chea left the courtroom after 20 minutes and was allowed to follow proceedings from a holding room. His co-defendant remained, although his eyes at times appeared to be closed.

Their complex case has been split into a series of smaller trials, starting with the forced evacuation of the population into rural labour camps and the related charges of crimes against humanity.

The closing statements in the first mini-trial started with lawyers for the “civil parties” that represent thousands of plaintiffs and will be followed by the prosecution and the defence.

Civil party lawyers alleged the defendants had intimate knowledge of the Khmer Rouge’s “elimination” policy through deliberate starvation and of the persecution of those branded “class enemies”.

Many of those deemed to be regime enemies still “face nightmares ... and constant mental suffering after witnessing torture and killing, including of their own relatives,” lawyer Sam Sokong told the court.

The statements are scheduled to be completed by the end of the month, with a verdict expected in the first half of next year.

“This is the final milestone before the verdict,” court spokesman Lars Olsen said.

“It is the last chance to convince the judges about the case.”

Other allegations, including genocide and war crimes, are due to be heard in later hearings although no date has yet been set.

Observers and survivors have long raised fears about the speed of proceedings and the advanced age of the accused.

Another defendant, former foreign minister Ieng Sary died aged 87 in March this year, while the case against his wife Ieng Thirith – also an ex-minister – was suspended after the court ruled dementia left her unfit to stand trial.

The court has been hit by delays caused by money shortages, staff walkouts, alleged political interference as well as the poor health of the accused.

In its historic first trial, the court in 2010 sentenced former prison chief Kaing Guek Eav to 30 years in prison – later increased to life on appeal – for overseeing the deaths of 15,000 people.

The court is investigating two possible new cases – strongly opposed by the government – against several lower-ranking cadres, although there are doubts about whether they will make trial.

Tens of thousands of people have attended the trial of Nuon Chea and Khieu Samphan.

It is a complex issue for the kingdom with many Khmer Rouge members returning to live within their communities after the regime’s fall.

Attending court for the closing statements, Kuy Pel, a 62-year-old farmer who lost several close relatives to starvation during the regime, expressed hope the trial will bring justice.